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308                 PA THOGENIC BA CTERIA.

The introduction of a fragment of the medulla ob-
longata of a dog dead of rabies beneath the dura mater
of a rabbit causes the development of rabies in the
rabbit in a couple of weeks. The medulla of this rabbit
introduced beneath the dura mater of a second rabbit
produced a more violent form of the disease in a shorter
time, and by frequently repeated implantations Pasteur
found that an extremely virulent material could be ob-

Inasmuch as the toxins of diphtheria and tetanus
circulate in the blood, and not infrequently saturate
the nervous systems of animals affected, it might be
concluded that the material with which Pasteur worked
was a toxin. This is readily disproven, however, not
only by the fact that the toxin would weaken instead of
strengthen by the method of transfer from animal to
animal, it not being a vital entity, but also by the dis-
covery that when an emulsion of the nervous system of
an affected animal is filtered through porcelain, or when
it is heated for a few moments to 100° C., or exposed
for a considerable time to a temperature of 75° or 80° C.,
its virulence is entirely lost. This would seem to prove
that that which is in the nervous system and communi-
cates the disease is a living, active body—a parasite, and
in all probability a bacterium. However, all endeavors
to discover, isolate, or cultivate this organism have failed.

Pasteur noted that the virulence of the poison was less
in animals that had been dead for some time than in
the nervous systems of those just killed, and by experi-
mentation showed that when the nervous system was
dried in a sterile atmosphere the virulence was attenu-
ated in proportion to the length of time it had been dry.
This attenuation of virulence of course suggested to
Pasteur the idea of a protective vaccination, and by in-
oculating a dog with much attenuated, then with less
attenuated, then with moderately strong, and finally with
strong, virus, the dog developed an immunity which
enabled it to resist the infection of an amount of viru-